The invention relates to a method and circuit for eliminating beam induced lag in a photoelectric tube, i.e., lag that is caused by the capacitance of the target and the effective resistance of the scanning beam, of such types of tubes. Typical of such photoelectric tubes are the Plumbicon (a trademark of N. V. Philips), Saticon (a trademark of Hitachi Denshi, Ltd.), video camera image pickup tubes.
By way of example, lag in such image pickup tubes is due to the resistance of the scanning beam and the capacitance of the target layer upon which the image is formed. The resulting finite time that it takes for the beam to charge the target layer gives rise to lag. It follows that in the design of sophisticated image pickup tubes, there are several operating parameters and structural configurations between which compromises must be made in order to provide the optimum tube efficiency. The primary compromise made in the design of, for example, Plumbicon tubes or Saticon tubes, is between the tube parameters known as resolution and lag. To illustrate, to provide an increase in the resolution of a tube and thus an increase in the image sharpness, the thickness of the target layer is reduced to minimize the scattering of light in the layer and the corresponding attendant loss of resolution. However, as the thickness of the layer is reduced, the capacitance increases, which increases the undesirable effects of lag during tube operation. Therefore, tubes presently are fabricated with compromised resolution because of the subsequent undesirable effects of lag. It follows that if lag can be eliminated, either by fabrication or structural techniques applied during manufacturing, or by electronic or operating techniques applied during use of the tube, then resolution could be maximized to provide tubes which would generate the sharpest possible video picture.
The traditional approach to the reduction of lag in camera pickup tubes has been, and still is, by means of what is commonly termed the bias light technique. Since lag is most evident in the darker areas of the picture the bias light is used to set a minimum light level. More particularly a lamp housed within the prism assembly of a television camera is used to evenly illuminate the faceplates of the pickup tube or tubes. However, the resulting video signal generated by the tubes is undesirably modified in accordance with the illuminating bias light.
It follows that additional circuitry must be employed in the video processing circuits to subtract the portion of the video signal generated by the bias light from the resulting modified video, to recreate the original scene viewed by the camera. Such circuitry is unduly complex and requires on the order of sixteen or more remotely controlled functions. Thus the bias light technique complicates the camera circuitry.
In addition, the bias light technique has the disadvantage of exaggerating the effect of unclean optics in the camera. Thus dirt or scratches, which normally would not be visible in the picture are, in effect, magnified by the use of bias light.
Typical of camera systems employing the light bias technique are those employed, for example, in the BCC-20 broadcast color camera manufactured by Ampex Corporation.
The invention overcomes the disadvantages of the bias light technique by providing a technique for reducing lag which is equally as effective, but which is significantly simpler to implement and control. Further, the invention allows a tube manufacturer to optimize the built-in resolution of the tube even though doing so provides tubes with prohibitive lag characteristics, since the undesirable effects of the increased lag are eliminated by the invention combination during operation of the tube.
The inventor has determined that for each value of charge stored on a specific section of the target of a photoelectric tube due to the action of incident light, there is a specific value of cathode voltage which ensures that that section of the target will be discharged to the dark value corresponding to the black level, in one pass of the scanning electron beam.
To this end, a voltage representative of the video output signal generated by a tube is supplied to a circuit having a selected, and preferably adjustable, transfer function characteristic such as typified, for example, by a logarithmic function or fractional power law curve as generated by gamma corrector circuits used in conventional video signal processing systems. The resulting non-linear signal is inverted and applied to a cathode of the tube via gain control means. Thus, in effect, the cathode is supplied with a feedback voltage whose value at any instant is proportional to the magnitude of the charge stored on the corresponding portion of the target being read out. Complete discharge can be effected with one pass of the scanning beam, resulting in the elimination of lag.
Accordingly, it is an object of the invention to provide a technique for eliminating lag in camera pickup tubes which is significantly simpler to implement than the prior art bias light technique, and gives rise to a generally cleaner black level.
Another object of the invention is to provide a selected voltage to the cathode of a photoelectric tube which ensures the complete discharge of charge generated by light incident on respective sections of the target in one pass of the scanning beam.
A further object is to supply a voltage to the cathode of a photoelectric tube which is an instantaneous value indicative of the charge on the respective portion, pixel, etc., of the target.
A still further object is to eliminate lag in a photoelectric tube by supplying a selected feedback voltage to the tube which is derived from the same or a different tube.
Another object is to eliminate lag in an image pickup tube via electronic means whereby the tube may be manufactured without a compromise between resolution and lag, to maximize the resolution of the tube.
Yet a further object is to provide a selected voltage to the cathode of a pickup tube to eliminate lag, wherein the voltage is conformed to the lag characteristic of the tube via a selected transfer function relationship.